The present invention relates generally to power electronic devices, and more particularly to drive circuits for MOS gated power electronic devices.
Metal Oxide Semiconductor (MOS) devices are constructed having a voltage controlled gate electrode. In operation, these devices are turned on by the application of bias (i.e., voltage) to the gate electrode. The gate provides capacitance to other electrodes (e.g., source and drain electrodes in a metal-oxide-silicon field-effect transistor) in these devices that need to be charged and discharged in order to turn the device on and off (i.e., charge is injected or extracted from the gate). In determining the operating requirements of these devices, the charge multiplied by the bias voltage represents a xe2x80x9cturn onxe2x80x9d and xe2x80x9cturn offxe2x80x9d energy.
In many power semiconductor applications, the device must be alternately turned on and off, often in conjunction with other devices, to thereby form a power conversion circuit. There is a desire to increase the frequency of operation (i.e., switching speed) of such circuits, which allows these circuits to be implemented with smaller and cheaper associated passive components. However, with the frequency of operation increased, the power required by the gate drive circuit increases proportionately (i.e., power equates to energy times frequency).
With respect specifically to gate drive circuits, half bridge circuits are commonly used either discretely or as part of a power MOS driver integrated circuit (IC). Using such a bridge type circuit, the gate is charged and discharged from a voltage source via the output resistance of the driver, the gate spreading resistance of the controlled device and any added series resistance. Thus, the forward and reverse passage, of charge through this resistance (i.e., the devices creating the resistance) results in a power loss. It should be noted that this charge flows as the driver switches, such that the driver will not necessarily be saturated, thereby resulting in higher resistance than the quoted xe2x80x9cfully onxe2x80x9d value of the driver. Reducing the resistance will not help reduce the loss as it just enables the same charge to flow more quickly.
In recent years, MOS gated power devices have replaced bipolar devices in many applications as a result of the advantages of MOS technology. For example, MOS gated power devices do not have static (i.e., DC) drive power consumption. In these devices, some AC power loss is acceptable, and at frequencies up to a few hundred kilohertz (KHz), this loss is often insignificant compared to other power losses in the circuit.
As frequencies are increased problems result. In particular, switching losses may increase and power dissipation in the switching device may also increase. In an attempt to address these problems, zero voltage, zero current and resonant switching techniques have been increasingly implemented in power conversion circuits. These circuits typically recirculate or recycle the energy involved in switching the device output capacitance, thereby reducing the power dissipation of the device and increasing overall efficiency.
Known devices, such as, for example, the MOS drive circuits shown in FIGS. 1 and 2, use a high Q (i.e., quality factor, which is a measure of the dissipation in a system) resonant circuit in the gate circuit to xe2x80x9cabsorbxe2x80x9d the gate capacitance, and generate a sinusoidal gate voltage. Essentially, a filter circuit (i.e., single element high Q tuned circuit in FIG. 1 and a four element filter circuit in FIG. 2) is provided to produce the sine wave signal. Specifically, in a typical half bridge configuration, a gate drive transformer is inserted between the resonant circuit and gate, with the transformer phased such that each device is driven with opposing phase. Correct switching of a driver device connected to the resonant circuit can eliminate most of the driver loss. However, only a very limited duty cycle is provided (i.e., at or about 50 percent).
Full sine wave resonant drive circuits provide limited control of duty cycle by varying amplitude, which affects the crossing point of the waveform and the threshold voltage of the driven device. In operation, reducing the drive voltage in order to significantly reduce the duty cycle will result in low amplitude past the gate threshold, and thus, poor saturation.
Therefore, known MOS drive circuits provide only very limited effective duty cycle operation, which is essentially 50 percent less the delays between the zero crossing and the gate threshold voltage. Reducing the amplitude will reduce the duty cycle, but also lengthen the switching time and reduces the xe2x80x9cpeak on bias.xe2x80x9d This will increase xe2x80x9cDC onxe2x80x9d losses.
In general, it is desirable to switch the controlled device off as fast as possible. As frequencies are increased, a greater portion of the switching period is required for the switching transition, thus requiring a shorter conduction time, which is shorter than can be achieved by known circuits.
Thus, there exists a need for a system having a drive circuit capable of effectively operating (i.e., no or nominal loss in power) over a greater range of duty cycles (i.e., about 25 percent to about 50 percent), and in particular, to such a system for driving gated power devices (e.g., MOS devices) over this greater range at higher frequencies. Such a system needs to control the duty cycle without requiring the reduction of source amplitude to unacceptable levels at these higher frequencies.
The present invention generally provides a half sine wave drive circuit and method of providing the same having independent adjustment of amplitude and duty cycle that recirculates or recycles the energy involved in switching the input capacitance of the driven device (e.g., MOS gated power device). Thus, the power dissipation of the device is reduced and overall efficiency increased, particularly at higher frequency operation (i.e., more than a few hundred KHz). In operation the present invention provides duty cycles of between about 25 percent and about 50 percent without loss of device power (i.e., no switching loss) at higher switching frequencies.
Specifically, a drive circuit for use in connection with a power device (e.g., MOS gated power device) includes switching means for providing effective operation over a greater duty cycle range, and a gate drive means for producing from a square wave input signal a half sine wave output signal for use as a drive voltage to the switching means to produce the greater duty cycle range. The gate drive means includes a resonant circuit capacitivly coupled to a gate of the switching means.
The switching means may be a transistor with the gate drive means providing the half sine wave output signal to the gate of the transistor. The resonant circuit is adapted to be configured between the operating frequency of the input signal and about twice the operating frequency to thereby provide a duty cycle of between about 50 percent and about 25 percent. Essentially, a driver inductance and capacitance of the resonant circuit are adapted to be configured to control the duty cycle based upon a drive frequency. The driver inductance of the resonant circuit may comprise a transformer, and in this construction, may be adapted to be configured to provide a duty cycle of between about 50 percent and about 75 percent.
A DC bias means further may be provided to control the DC level of the circuit. The resonant circuit may comprise, for example, a class E single ended resonant circuit capacitivly coupled to the gate of the switching means. A hard switch, such as a logic gate, may be included for providing the input signal. The relatively low power required at this input results in the power lost, and hence dissipation, in the logic gate to not be excessive.
In another embodiment, a resonant drive circuit of the present invention providing improved independent amplitude and duty cycle control without loss of power at higher operating frequencies includes a MOS controlled device (e.g., transistor) to be switched to provide a duty cycle of between about 25 percent and about 50 percent. A resonant drive circuit capacitivly coupled to a gate of the MOS controlled device is also included and provides from a square-wave input signal, a half sine wave output signal for driving the MOS controlled device. The resonant drive circuit is adapted for operation between an operating frequency of a driven device (e.g., MOS gated power device) and about twice the operating frequency, defined by the input signal.
A driver inductance and capacitance of the resonant drive circuit are configurable to provide the duty cycle of between about 25 percent and about 50 percent. A DC bias may be provided to control the DC level of the half sine wave signal. The resonant circuit may include a transformer for providing the driver inductance. A hard switched input device may be included for providing the input signal.
A method of the present invention for controlling the amplitude and duty cycle of a drive circuit without losing power at higher operating speeds includes receiving a square wave input signal, producing a half sine wave output signal from the square wave input signal using a resonant circuit capacitivly coupled to a switching a device, and configuring the resonant circuit to operate at a duty cycle of between about 25 percent and about 50 percent. DC bias also may be provided to the resonant circuit.
The method further may include recycling through the resonant circuit a gate charge of the device being switched. A transformer also may be used in connection with the resonant circuit to add isolation or invert the drive waveform for 50 to 75 percent duty operation.
Thus, the present invention provides a resonant drive circuit and method of providing the same that is capable of operation over a greater duty cycle range (i.e., from about 25 percent to about 50 percent) without experiencing power loss at higher frequencies. Through the adjustment of components within the resonant circuit, an appropriate duty cycle may be provided for driving a switching device, such as, for example, a MOS gated power device.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.